Joint Precoder and Artificial Noise Design for MIMO Wiretap Channels With FiniteFinite Alphabet Inputs Based on the Cut-Off Cut Rate
Abstract: We consider precoder and artificial noise (AN) design for multi-antenna multi wiretap channels under the finite-alphabet finite alphabet input assumption. We assume that the transmitter has access to the channel coefficients of the legitimate receiver and knows the statistics of the eavesdropper's channel. Accordingly, we propose a secrecy rate maximization algorithm using a gradient descentdescent-based optimization of the precoder matrix and an exhaustive search over the power levels allocated to the AN. We also propose algorithms to reduce the complexities complexi of direct ergodic secrecy rate maximization by: 1) maximizing a cut cut-off rate-based approximation for the ergodic secrecy rate, simplifying the mutual information expression, which lacks a closed-form closed form and 2) diagonalizing the channels toward the legitimate mate receiver and the eavesdropper, which allows for employing a per pergroup precoding-based based technique. Our numerical results reveal that jointly optimizing the precoder and the AN outperforms the existing solutions in the literature, which rely on the preco precoder der optimization only. We also demonstrate that the proposed low complexity alternatives result in a small loss in performance while offering a significant reduction in computational complexity.